The Soviet Venus exploration program extended over more than two decades from the first attempt to send a spacecraft to Venus to the program’s closure. At the program’s inception, scientists underestimated the atmospheric and surface conditions, making the initial missions underprepared for the challenges of conducting scientific investigations. While later missions benefited from returned science and were designed with increased capabilities to deal with the severe environmental conditions, in some cases spacecraft design was unable to keep pace with the rapidly increasing science understanding and missions were launched with only partially effective protection systems.

The Soviet program began with the Venera 1 probe. After 1961, the Soviet RKA developed a new spacecraft architecture consisting of an orbital module connected to a descent probe or lander for in situ missions, or to an instrument module for orbital or flyby missions. This architecture was implemented in all Venus missions from Zond 1 (1964) up to Venera 8 (1972). After the initial attempt with Venera 1, the Zond 1 impactor and the Venera 2 (1965) probe soon followed, but didn’t succeed due to telecom system failure. The next step was the Venera 3 probe (1965), the first man–made object to land on another planet.

During the late 1960s, the Soviet RKA continued to improve both the Venus orbiting module and descent capsule. At the time, it was thought that the surface temperature of Venus was approximately 300C, with an atmosphere consisting mainly of carbon dioxide and nitrogen at about 20 bars. Consequently, the capsule was designed to survive 300C and 25 bars. Venera 4, launched in 1967, reached Venus and transmitted data on the atmosphere and environment until it reached an attitude of about 26 km after 93 minutes of descent. Venera 5 and Venera 6 were launched in 1969 within a week of one another. Both were equipped with a set of sensors to further characterize the Venus atmospheric temperature, pressure, and composition. Because they were designed to survive 25 bars, they were not expected to deliver any surface data. Although mission designers were aware of the new analysis of the Venus environment, it was too late to significantly redesign the landing capsule without missing the next launch opportunity. However, based on the experience with Venera 4, the parachutes were modified to accelerate the capsule’s descent, allowing it to reach greater depths prior to overheating the capsule’s electronics. Both Venera 5 and 6 survived the descent for about 50 minutes and reached altitudes of approximately 20 km, where the pressure exceeded 27 bars and temperature exceeded 300C. Their failures were most likely due to crushing of the pressure vessel.

Venera 7 Landing Capsule

Venera 13 mockup at the Cosmos Pavillion in Moscow.

The next series of missions, Venera 7 (1970) to 10 (1975), represented a leap forward for in situ science as the first data were returned from another planet. These spacecraft were designed to sufficiently mitigate the challenging conditions experienced during descent through the Venus atmosphere, as well as on the surface. With Venera 9 (1975) and 10, the Soviet space agency introduced a completely redesigned spacecraft, used for all later Venus missions. Venera 9 and 10 transmitted data from the surface for 53 and 65 minutes, respectively. The landers’ capabilities were not the limiting factors in the surface survival time; instead, each mission terminated when its orbiter exited the communication range.

The next series of landers, Venera missions 11, 12, 13, and 14, (all launched in 1978 within a few days from each other) improved upon the successes of the ongoing Soviet Venus exploration program. These missions all descended to the surface in approximately one hour and lasted on the surface for up to two hours. The Venera 11 and 12 spacecraft were quite similar to Venera 9 and 10, with a few modifications and new instruments. They descended through the clouds and atmosphere to reach the surface in about one hour. Venera 11 transmitted data for about 95 minutes, until the flyby spacecraft used as relays were out of reach. Venera 12 transmitted data for 110 min. Unfortunately, not all the experiments on Venera 11 and 12 succeeded. Venera 13 was very similar to Venera 12, except for increased battery power and more instruments. Venera 13 included some aerodynamic modifications that increased the lander’s stability during free fall. The Venera 13 and 14 landers featured an improved sample acquisition system. Venera 13 transmitted for 127 min and Venera 14 for 57 min to the relay bus, until the relay moved out of the lander’s communication range.

The Venera 13 Lander image of the surface of Venus at 7.5 S, 303. E, east of Phoebe Regio.

The last Soviet mission to the surface of Venus, Vega, launched in 1984 with two identical spacecraft containing a lander and a balloon. This mission combined a Venus swingby and a Comet Halley flyby (thus the name Venera–Gallei, or Vega). Two identical spacecraft, Vega 1 and Vega 2, were launched December 15 and 21, 1984, respectively. After carrying Venus entry probes to the vicinity of Venus, the two spacecraft were retargeted using Venus gravity field assistance to intercept Comet Halley in March 1986. The Vega architecture was similar to Venera 14, but it included new instruments focused on measuring the composition and size distribution of cloud particles, as well as on the direct detection of sulfuric acid. Each of the two Vega entry systems carried a Venera-like lander and a 3.4 m diameter superpressure balloon. The balloons were designed to deploy and float high in the atmospheric clouds (~53 km) where the temperature was near that of Earth. The balloons traversed ~12000 km and stopped transmitting after approximately 48 hrs of operation, after exhausting their batteries.